Currently, the relationship between the monopole and the circuit is that the transmitter radiates the electromagnetic wave out by a whip-like antenna. However, the whip-like antenna is a stick-shaped or strip-shaped object protruding the electronic device, like the antenna disposed on the past cellphone, router or modem. Therefore, the whip-like antenna occupies much space and is easy to be damaged. Hence, due to the demands of the miniaturization and simple appearance, the built-in antenna becomes the mainstream for most of the current radio products, especially the consuming electronic products.
Please refer to FIG. 1, which shows a conventional radio device. The conventional radio device includes a circuit board 1 having an electronic element area 10. A transmitter 11 is disposed in the electronic element area 10, and a main radiator 20, i.e. an antenna, is disposed on the circuit board 1 outside the area 10 by the printed circuit board technology. The main radiator 20 is electrically connected to the transmitter 11 via the circuit (not shown) in the circuit board 1 so that the electromagnetic wave can be radiated out. However, since the antenna 20 is disposed on the circuit board 1 with electronic elements in the electronic element area 10, the mutual interference between the antenna 20 and the electronic elements is easy to occur. Besides, the electronic elements also hinder the electromagnetic wave from being transmitted. Therefore, the efficiency of the antenna 20 is a little poorer than that of the conventional whip-like antenna.
Please refer to FIG. 2, which is a gain diagram in the prior art. Since the shape of the antenna integrated with the circuit board is not as perfect as that of the conventional whip-like antenna, FIG. 2 is made by using the plane perpendicular to the major axis direction of the antenna as a measuring plane. That is, FIG. 2 is a polar coordinate diagram drawn by using the antenna as the center of a circle. It can be clearly seen from FIG. 2 that from 90° to 270°, the radiation strength of the antenna 20 of FIG. 1 is weakened and not fixed. Taking the user of the wireless modem for example, when he installs the modem, he cannot use the wireless transmission at once; contrarily, he has to adjust the posture of the modem often to enable the angle of the modem having a better radiation strength of the electromagnetic wave to be aimed at the receiving device, e.g. the notebook computer. However, the wireless modem itself has to be connected to the signal source in a wired way, and needs the supply of power. Therefore, the adjustment for the posture of the wireless modem is restricted to the substantial wire. Moreover, if more than two receiving devices respectively at different positions need to use the wireless modem simultaneously, the application is extremely inconvenient. Furthermore, if the wireless modem is fixed on the ceiling or wall, the posture thereof cannot be adjusted at will. Hence, in order to solve the issue of the uneven signal strength, the current technologies all focus on the shape of a single antenna. However, the effect is limited. Otherwise, the way of enhancing the power is performed to solve the issue of the weakened strengths of some angles. However, this not only interferes other external electronic devices but also consumes more power.
In order to overcome the drawbacks in the prior art, an antenna device for a circuit board is provided. The particular design in the present invention not only solves the problems described above, but also is easy to be implemented. Thus, the present invention has the utility for the industry.